You have some problems with your code like Sql Injection, you can fixed with parameterized queries.

You are not taking the Text property of your textbox1. Fix your naming and don't use textbox1 for control names. Naming is important, so you can be understand by other programmers. Same is true for your database table, Table1 is not appropriate name.

You don't need to Close() the connection when you are in using block. Using do this automatically. Also when you create an SqlConnection object you can open a connection, there is no need to make if checks about it.